1. Field of the Invention
The present invention relates to a semiconductor device.
2. Related Background Art
A power semiconductor element such as a switching element or a diode is used in circuits such as a switching power supply and an inverter and desired to have such properties as a high breakdown voltage and a low turn on-resistance.
However, the breakdown voltage and the turn on-resistance are in a relationship of trade-off, which is determined by a material of the element. The main element material, silicon, has already reached almost its limits of low turn on-resistance by the past progresses in technological development. Therefore, in order to further lower the turn on-resistance, any other element material must be used.
Specifically, use of a nitride semiconductor such as GaN or AlGaN or a wide bandgap semiconductor such as silicon carbide (SiC) as the switching element material has improved the trade-off relationship determined by the element material. Thus, the turn on-resistance is dramatically lowered (see, for example, Japanese Patent Laid Open Pub. 2005-093864). It is to be noted that the elements which are made of a nitride semiconductor such as GaN or AlGaN and whose turn on-resistance can be easily lowered may include, for example, a hetero-structure field effect transistor (hereinafter referred to “HFET” simply) having an AlGaN/GaN hetero-structure. The HFET can be of low turn on-resistances because of a high mobility in hetero-structure interface channels and a high electron density due to piezoelectric polarization.
However, since the HFET is a horizontal type element, when it is supplied with a high voltage between its gate and drain, an electric field is concentrated to an end portion of the gate. If the electric field may be intensified due to the concentrated electric field, electrons accelerated by the high electric field are injected into a passivation film or an AlGaN layer, thus generating crystal defects. This results in a problem in that the properties fluctuate and the reliabilities of the element deteriorate.
Such deteriorations in reliability caused by a high electric field are suppressed effectively by relaxing the electric field concentration. Electric field relaxation may be realized by a field plate structure (see, for example, Japanese Patent Laid Open Pub. 2005-093864). Further, by forming a plurality of field plate electrodes and changing the thickness of insulating films under those field plate electrodes in such a manner that it may increase step-wise as it approaches the side of a drain, the number of the positions where electric field is concentrated increases, thus enabling lowering a peak of the electric field.
However, this method has a disadvantage in that a larger number of times of depositing insulating films may increase fluctuations in properties due to variations in their film thicknesses. Furthermore, this method may raise a problem in that, if the insulating films become too thick, a film stress is increased and thus cracks generate in the insulating films, resulting in deteriorations in reliability.